There are two Global Navigation Satellite Systems (GNSS) which have been fully deployed for a number of years (the US Global Positioning System, the Russian GLONASS) and two more which are under deployment (the Chinese Beidou Navigation Satellite System and the European Galileo system). These systems rely on the same principles: microwave radio signals are broadcast from a number of satellites which orbit in a non-geostationary orbit; the signals carry a PRN code which is correlated with a local replica in a receiver configured to receive the broadcast signals; when a receiver is capable of acquiring and tracking signals from a satellite, its processing capabilities demodulate the code signal by using the correlation process and calculate a pseudo-range, which is the distance between the receiver and the satellite (affected by various error sources). This pseudo-range is taken in combination with pseudo-ranges acquired from other satellites (generally three) to determine a position, velocity and time (PVT) solution.
Radio navigation signals transmitted by some satellites are known as BOC signals (Binary Offset Carrier modulation), where a carrier wave is first modulated by a PRN code, and then by a subcarrier. The resulting signal has a spectrum having two main lobes located on either side of the carrier frequency, thus allowing cohabitation with other signals using the same carrier frequency. BOC signals are referred to as BOC (m, n), where the chip rate of the code signal is n*1.023 Mcps (Mega Chips per second), and the subcarrier frequency is m*1.023 MHz.
The tracking of BOC signals has proven to offer more precise and robust positioning information than PSK signals, mainly thanks to the sharper slope of its autocorrelation function peak, and its bandwidth. However, unlike PSK signals, the autocorrelation function of a BOC signal shows several side peaks which compete with the main peak, some of the side peaks having magnitudes close to the main peak magnitude.
The correlation of the received BOC signal with a reference signal is used to construct a discriminator value for controlling a Phase Locked Loop (PLL) of the receiver and tracking the positioning signal. A PVT is then constructed using the phase of the PLL, and a received navigation message.
When error sources, such as noise or interferers, affect the received signal, the PLL might lock its tracking position on one of the side peaks of the correlation between the received signal and a reference signal, which leads to the introduction of a bias in the position measurements. This bias can range from ˜9.7 meters for a BOC (15, 2.5) signal, to ˜146.5 m for a BOC (1,1) signal. BOC signal receivers must deal with this issue to ensure a precise positioning.
Positioning signals are also affected by multipath, due to reflections on the environment occurring during the signal propagation. These multipath reflections are particularly present when operating in an urban or indoor environment. The reception of multipath signals creates artifacts in the correlation function between the received signal and the reference signal, multipath peaks being shifted from the original peaks by a distance corresponding to the delay between the main path and the multipath.
A number of state of the art techniques are dealing with the issue of synchronizing the tracking on the main peak of a BOC signal, but these techniques do not sufficiently consider propagation in a multipath environment.
Among these techniques, the Bump-Jumping technique, described in U.S. Pat. No. 8,964,813, is based on performing two additional correlations with a delayed version of a reference signal, a Very-Early (VE) correlation and a Very-Late (VL) correlation, and detecting tracking on side peaks of the BOC signal by comparing the energy of the Prompt, Very-Early and Very-Late signals. When the Prompt correlation is detected as not having the highest amplitude, a phase jump is operated in the direction corresponding to the highest correlation amplitude. Due to the direct estimation of the correlation amplitude over the Prompt, Very-Early and Very-Late positions, this technique is particularly sensitive to multipath reflections.
Another technique, known as the Double Estimation technique, is described in European Patent EP 2 049 914 B1. This technique implements two tracking loops, a C-DLL (for Code Delay Lock Loop), tracking the PRN code of the BOC signal, and a S-DLL (for Subcarrier Delay Lock Loop), tracking the subcarrier of the BOC signal. The difference between the pseudo ranges estimated within the two loops is used to readjust the measured pseudo-range. As multipath reflections might affect differently the C-DLL and the S-DLL loops, false readjustments of the subcarriers may occur frequently, leading to false pseudo range measurements.
Another technique, known as the Double-Discriminator technique, is described in European Patent EP 2 382 484 B1. This technique involves calculation of two discriminators in parallel, based on the subcarrier and code of the BOC signal. The first discriminator calculation, called a non-ambiguous discriminator calculation, leads to a non-ambiguous determination of a tracking position, unlike the second discriminator calculation, which is ambiguous. However, the second discriminator calculation is more precise and less sensitive to noise and multipath reflections than the first one. A selection unit is configured to compare the value of the first discriminator with a threshold, and to select the discriminator value used in the tracking loop depending on the result of this comparison. Compared to the previous techniques, this technique behaves well in case of false alarms, because it does not create abrupt code phase jumps to readjust the phase of the tracking loop when detecting a tracking position on a side peak. However, because multipath reflections significantly affect the shape of the non-ambiguous discriminator values, they can preclude detection of side peak tracking, which leads to wrong pseudo range measurements.